The field of hydrocarbon production is directed to retrieving hydrocarbons that are trapped in subsurface reservoirs. Typically, these reservoirs are comprised of parallel layers of rock and fluid material each characterized by different sedimentological and fluid properties. Hydrocarbons accumulate below or between non-porous or lower permeability rock layers, forming reservoirs. These hydrocarbons can be recovered by drilling wells into the reservoirs. Accordingly, hydrocarbons are able to flow from the reservoirs into the well and up to the surface. The production rate at which hydrocarbons flow into the well is vital to the petroleum industry and as a result, a large amount of effort has been dedicated to developing techniques in order to better predict fluid flow and geomechanical characteristics of subsurface reservoirs. One of these techniques relates to gridding of a reservoir, which will be discussed later in more detail herein.
Highly complex geological subsurface reservoirs, such as reservoirs having a network of fractures, present unique and specialized challenges with regards to reservoir simulation. Subsurface reservoirs with a network of fractures typically have a low permeability rock matrix, making it difficult for hydrocarbons to pass through the formation. Fractures can be described as open cracks or voids within the formation and can either be naturally occurring or artificially generated from a wellbore. The presence of fractures, can therefore play an important role in allowing fluids to flow through the formation to reach a well. For example, hydrocarbon production rates from a well tend to be very different depending on whether the well is intersected by a large fracture. Sometimes fluids such as water, chemicals, gas, or a combination thereof, are injected into the reservoir to help increase hydrocarbon flow to the production well. In situations in which a fracture provides for direct connectivity between a production well and a fluid injection well, the injected fluids can flow through the fracture and bypass the majority of hydrocarbons within the formation that the injected fluids were supposed to help produce. Therefore, it is desirable to characterize the extent and orientation of fractures in hydrocarbon reservoirs to properly forecast geomechanical and fluid flow characteristics through the subsurface formation. In order to compute these characteristics, one must first apply gridding techniques.
Reservoir gridding techniques can be described as the process of decomposing a 3D reservoir volume into a plurality of smaller and simpler 3D volumes, which are typically convex 3D volumes. Accordingly, these techniques break a continuous simulation domain into discrete counterparts that can subsequently be used to construct a simulation model by discretizing the governing equations describing fluid flow, heat transfer, geomechanics, or a combination thereof. Within the reservoir simulation community, the discrete volumes are typically referred to as cells, finite volumes, control volumes, or finite elements depending on the discretization and simulation techniques being utilized.
For fractured subsurface reservoirs, gridding poses a unique challenge due to the geometric complexity and stochastic nature of the network of fractures. For instance, gridding strategies for conventional reservoir simulations are typically not designed to handle a large number of internal geometric features, such as fractures. Many gridding strategies are only suitable for geometries that are well connected and do not contain cracks or overlaps, which are often referred to as “water-tight” geometries. For example, these gridding strategies typically pre-compute the intersections of all geometric features. Others gridding strategies are unable to achieve good grid quality while capturing certain geometric intricacies, such as one fracture slightly penetrating the plane of another fracture, two fractures being close to one another without intersecting, or two fractures intersecting with a small dihedral angle. In general, grids containing internal features must balance the opposing goals of accurately approximating the features and maintaining good quality. If reservoir gridding techniques are not able to accommodate these fine-details while maintaining good grid quality, simulation of the model can result in reduced accuracy, increased run times, convergence problems, or a combination thereof.
The prior art has attempted to improve grid quality by preprocessing fracture sets to remove the most problematic configurations. However, one skilled in the art will recognize that this is not straightforward as removal of one problem often creates another. In addition, the implementation of these past attempts is nontrivial due to considerations of floating-point arithmetic in calculating fracture intersections.